As an organic polymer film conventionally used as an interlayer insulating film for a VLSI, an organic polymer film formed through polymerization of molecules including an aromatic group for increasing heat resistance, such as an organic polymer film made of a polyimide derivative, a polyallyl ether derivative, a polyquinoline derivative or a polyparaxylene derivative, is known. Since such an organic polymer film includes carbon as a principal constituent, the polarizability of constituent molecules is small and hence the dielectric constant is low as compared with that of a conventional interlayer insulating film including SiO2 as a principal constituent. Therefore, such an organic polymer film is regarded as an interlayer insulating film with a low dielectric constant. The dielectric constant of an organic polymer film including carbon as a principal constituent is approximately 2.4 through 3.0, which is lower than the dielectric constant of a general interlayer insulating film including SiO2 as a principal constituent, that is, approximately 3.3 through 4.5. However, some of the interlayer insulating films including SiO2 as a principal constituent, such as an organic SOG including an organic constituent, are known to have a dielectric constant of approximately 2.9.
The conventional organic polymer film attains a low dielectric constant because the polarizability of organic molecules is smaller than that of SiO2, and in order to further lower the dielectric constant, examination is being made on porosity of an organic polymer film.
When an organic polymer film is made porous, however, although the dielectric constant can be largely lowered, the adhesion and the mechanical strength are lowered on the other hand. This is because of a principle defect that the lowering of the dielectric constant resulting from the porosity is realized through lowering of the crosslink density of the organic polymer film. Since the mechanical strength of an organic polymer film is higher as the crosslink density is higher, the lowering of the crosslink density resulting from the porosity lowers the hardness of the organic polymer film as well as its glass transition temperature.
The lowering of the adhesion and the mechanical strength of an interlayer insulating film can disadvantageously cause destruction of the interconnect structure in chemical mechanical polishing (CMP) performed for planarizing the interlayer insulating film. Also, when the glass transition temperature is lowered, the interlayer insulating film is softened in annealing subsequently carried out on the interlayer insulating film, and hence, the multilayer interconnect structure can be disadvantageously deformed or destroyed.
In order to realize a low dielectric constant of an organic polymer film without lowering the adhesion and the mechanical strength, Japanese Laid-Open Patent Publication No. 2001-332543 proposes a low dielectric organic polymer film with a three-dimensional polymer structure in which three-dimensional pores are formed in an organic polymer. In a method described in this publication, a polymer structure including three-dimensional pores with a size of a molecular level is formed by, for example, forming a copolymer of a three-dimensional crosslinked molecule having four functional groups and a straight chain molecule (two-dimensional crosslinked molecule) having two functional groups that form a chemical bond with the functional groups of the three-dimensional molecule or by polymerizing molecules having pores among them.
In this method, a pore with a size of a molecular level can be formed in an organic polymer film, and therefore, improvement of heat resistance (improvement of the glass transition temperature) derived from the increased crosslink density and improvement of the mechanical strength can be attained at the same time as a low dielectric constant.
An organic polymer film with a high crosslink density, however, has a high level three-dimensional network, and hence is likely to gel. A gel is a wet solid in which a large amount of solvent is insolubilized in the network as a result of crosslinkage proceeded as a polymerization reaction proceeds.
Therefore, it is very difficult to form a mold or adjust a raw material suitable to a thin film from a gel wet solid and it is impossible to apply a gel wet solid on a substrate.
Accordingly, it is very difficult to form an organic polymer film that includes pores with a size of a molecular level and has a high crosslink density.